1. Field of the Invention
The subject invention is generally related to compounds which are capable of allosterically modifying hemoglobin and, more particularly, to a family of new compounds which, when mixed with blood, interact with hemoglobin to drive the allosteric equilibrium toward a low oxygen affinity state.
2. Description of the Prior Art
Hemoglobin is a tetrameric protein which delivers oxygen via an allosteric mechanism. Oxygen binds to the four hemes of the hemoglobin molecule. Each heme contains porphyrin and iron in the ferrous state. The ferrous iron-oxygen bond is readily reversible. The first oxygen bound to a heme requires much greater energy than the second oxygen molecule, binding the third oxygen requires even less energy, and the fourth oxygen requires the lowest energy for binding. Hemoglobin has two .alpha. and two .beta. subunits-arranged with a two fold symmetry. The .alpha. and .beta. dimers rotate during oxygen release to open a large central water cavity. The allosteric transition that involves the movement of the alpha-beta dimer takes place between the binding of the third and fourth oxygen. The .alpha..sub.1 .beta..sub.1 interface binding is tighter than the .alpha..sub.1 .alpha..sub.2 or .alpha..sub.1 .beta..sub.2 interfaces.
In blood, hemoglobin is in equilibrium between two allosteric structures. In the "T" (for tense) state, hemoglobin is deoxygenated. In the "R" (for relaxed) state, hemoglobin is oxygenated. An oxygen equilibrium curve can be scanned, using well known equipment such as the AMINCO.TM. HEM-O-SCAN, to observe the affinity and degree of cooperativity (allosteric action) of hemoglobin. In the scan, the Y-axis plots the percent of hemoglobin oxygenation and the X-axis plots the partial pressure of oxygen in millimeters of mercury (mm Hg). If a horizontal line is drawn from the 50% oxygen saturation point to the scanned curve and a vertical line is drawn from the intersection point of the horizontal line with the curve to the partial pressure X-axis, a value commonly known as the P.sub.50 is determined (i.e., this is the pressure in mm Hg when the scanned hemoglobin sample is 50% saturated with oxygen). Under physiological conditions (i.e., 37.degree. C., pH=7.4, and partial carbon dioxide pressure of 40 mm Hg), the P.sub.50 value for normal adult hemoglobin (HbA) is around 26 mm Hg. If a lower than normal P.sub.50 value is obtained for the hemoglobin under test, the scanned curve is considered to be "left-shifted" and the presence of high affinity hemoglobin is indicated. Conversely, if a higher than normal P.sub.50 value is obtained for the hemoglobin under test, the scanned curve is considered to be "right-shifted" and the presence of low affinity hemoglobin is indicated.
It has been proposed that influencing the allosteric equilibrium of hemoglobin is a viable avenue of attack for treating diseases. The conversion of hemoglobin to a high affinity state is generally regarded to be beneficial in resolving problems with deoxy Hemoglobin-S (sickle cell anemia). The conversion of hemoglobin to a low affinity state is believed to have general utility in a variety of disease states where tissues suffer from low oxygen tension, such as ischemia and cancer. Several synthetic compounds have been identified which have utility in the allosteric regulation of hemoglobin and other proteins. For example, several new compounds and methods for treating sickle cell anemia which involve the allosteric regulation of hemoglobin are reported in U.S. Pat. No. 4,482,571 to Abraham, U.S. Pat. No. 4,699,926 to Abraham et al., U.S. Pat. No. 4,731,381 to Abraham et al., U.S. Pat. No. 4,731,473 to Abraham et al., U.S. Pat. No. 4,751,244 to Abraham et al., and U.S. Pat. No. 4,887,995 to Abraham et al. Furthermore, in both Perutz, "Mechanisms of Cooperativity and Allosteric Regulation in Proteins", Quarterly Reviews of Biophysics 22, 2 (1989), pp. 163-164, and Lalezari et al., "LR16, a compound with potent effects on the oxygen affinity of hemoglobin, on blood cholesterol, and on low density lipoprotein", Proc. Natl. Acad. Sci., U.S.A. 85 (1988), pp. 6117-6121, compounds which are effective allosteric hemoglobin modifiers are discussed. The structure of certain other synthetic compounds which decrease the oxygen affinity of hemoglobin is reported in the abstract by Randad et al., "Allosteric modifiers of hemoglobin. Synthesis and Testing of Novel compounds to decrease the oxygen affinity of hemoglobin" American Chemical Society Division of Medicinal Chemistry, 200th ACS National Meeting, Item #71, Washington D.C., Aug. 26-31, 1990; however, no data is provided with the abstract.